Photonics is the science and technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of technical applications of photonics to devices extends from energy generation to detection to communications and information processing and storage. In addition to its other properties, the polarized nature of light can be exploited to expand or improve the efficiency, utility, and specificity of photonics devices.
Organic based photonics devices have been under development for more than 12 years and offer many potential advantages and opportunities for improved devices. For example, organic electroluminescence (the emission of light in response to an electrical current) has been used in display technology. In such a device, organic materials which posses the ability to emit light when electric current is passed through them are organized as thin layers between two electrodes. The emitters used in light emitting diodes (LEDs) such as these can be either small organic molecules or conjugated polymers.
Organic materials are useful for other photonics devices as well. For example, the need to develop efficient low-cost photovoltaic devices (devices that convert light into electrical energy) has stimulated research efforts using organic materials as or as part of the photoactive media. Photovoltaic devices based on organic materials such as organic molecules and conjugated polymers are emerging as an alternative technology to more conventional approaches based on inorganic semiconductors. Compared to inorganic semiconductor counterparts, organic materials offer the advantages of high photosensitivity, high optical absorption coefficients, and compatibility with vacuum deposition, thereby possessing the potential for large area, thin-film devices that can be produced at a modest cost. Furthermore, organic materials can be deposited on flexible or shaped substrates, which may eventually lead to the development of lightweight and conformal devices.
One fascinating feature of organic materials is their potential for controlling macroscopic material properties by manipulating the order or orientation of the molecules. For example, alignment of an organic material along a given axis can yield preferential absorption and emission along that axis. LEDs using aligned photoactive materials which can emit polarized light will be particularly useful as backlights for conventional liquid crystal displays (LCDs), since in these systems 50% of the emission of an unpolarized light sources is typically lost due to polarization based filtering. In addition, control of the alignment of emissive molecules in LEDs is quite important for future advancement in emission devices, such as LEDs integrated with microcavities and waveguide structures.
Research efforts directed at achieving polarized electroluminescence by aligning the organic emitters have been reported. However, all of these approaches are non-general, have difficulty controlling film thickness and uniformity, and are time consuming. For example, there is the liquid crystal approach. Devices based on this method are inherently limited to being made from materials that are liquid crystals. Epitaxial growth on rubbed substrates is another approach that has been attempted. This method is only applicable to use with small organic molecules which can be vapor deposited. An elongation approach has also been tried; but this method is complicated and film thickness and uniformity are difficult to control. The Langmuir-Blodgett method has also been used to develop polarized electroluminescent devices; but it has limited applicability and is specifically limited to materials that are amphiphilic and are capable of forming Langmuir-Blodgett films.
Electronic organic devices developed using a poly(tetrafluoroethylene) (PTFE) oriented film as a template to provide alignment and orientation of subsequently deposited films have been reported. See Katsuya Wakita, U.S. Pat. No. 5,546,889. Such devices are, however, fundamentally limited to electronic devices such as field effect transistors because, among other reasons, the electrodes used in these devices are necessarily co-planar and hence are inapplicable to photonics devices. Moreover, because the electrodes are co-planar, it is not feasible to prepare multiple stacked layers between the electrodes. Furthermore, Wakita is limited to purely electronic devices and does not enable photonics devices since it provides for neither photoactive materials nor for any electrodes to be transparent. Absent these and other features, such a device is not suitable for photonics applications. The electronic device developed by Wakita is also unsuitable for photonics applications because it fails to overcome the problem of charge conduction through the PTFE alignment layer, which is electrically insulating. That is, it fails to answer the question of how to use a polymer, such as PTFE, for alignment without completely insulating charge conduction in an organic electronic device.
To date, polarization sensitive organic photovoltaic devices have not been reported.
For the foregoing reasons, there is a need for polarized organic photonics devices. Moreover, there is a need for a processing method that is simple and fast, applicable to a variety of organic and polymeric materials, yields high optical quality films, and easily achieves thickness of a few tens of nanometers.
The present invention is directed to polarized organic photonics devices, and process for production thereof, that satisfies the need for polarized organic photonics devices, and processes for production thereof, as well as other needs.
The process for fabricating a polarized organic photonics device beings with preparing a alignment layer on top of a first conducting layer or conducting substrate. The first conducting layer or conducting substrate serves as a first electrode in the photonics device. The alignment layer, typically a thin layer of an insulating, electron transporting or hole transporting material, is deposited by a friction transfer method. This layer provides for the alignment of subsequently deposited organic and polymeric layers, necessary for polarized emission and absorption. Following the alignment layer, a conducting polymer may be deposited onto the alignment layer. This step may be carried out at elevated temperatures to enhance the uniformity of the deposited layer. Next, a photoactive material is deposited. As used herein, a photoactive material is a material that interacts with or emits light. This step may also be carried out at elevated temperature, to enhance uniformity and to further increase the alignment of the photoactive material to the preferred direction defined by the alignment layer. Finally, a second conductive layer is added to yield a polarized organic photonics device. The second conductive layer serves as a second electrode in the photonics device.
Specific advantages of the present invention include, among others, the following:
(i) A simple method of generalized applicability for creating polarized organic photonics devices.
(ii) Organic photonics devices with enhanced efficiency due to the polarized response of aligned photoactive material.
(iii) Organic photonics devices with enhanced selectivity due to the polarized response of the aligned photoactive material.
(iv) The ability to simultaneously align organic or polymeric species using a layer of a alignment material that may be electrically insulating without eliminating charge conduction through the alignment layer.